In the works: A formula to calculate the level of risk at which to start bone drugs

In the decade or so since the World Health Organization (WHO) first characterized the terms osteoporosis and osteopenia, basing them on bone-density measurements from dual-energy x-ray absorptiometry (DXA), we have come to know the definitions well, thanks to attention in the lay and medical press (TABLE 1).1

What is the goal behind the heightened public awareness? To reduce the number of osteoporotic fractures.

Do the WHO definitions further this goal? Not really.

Rather than an isolated numerical value, a more useful and descriptive definition of osteoporosis is the following: a skeletal disease characterized by low bone mass and disruption of bone tissue architecture that reduces the mechanical strength of the skeleton and increases the risk of fragility fractures.

In many cases, this definition would encompass women now diagnosed as having osteopenia. Unfortunately, although risk factors for osteoporosis have been described and well promulgated (TABLE 2), the almost exclusive focus of diagnosis has been and continues to be DXA scanning and the WHO definitions, with their reliance on the term osteopenia to convey heightened risk short of full-blown osteoporosis.

This article explains why that way of assessing a woman’s risk of fracture is not the most informative. In fact, the term osteopenia has very little clinical relevance. Some women in the osteopenic range have a high risk of fracture in the short term, whereas others have a great deal of bone health. The hope is that this term will be retired in the near future and replaced with tools that enable us to calculate the absolute fracture risk in 5 to 10 years.

Bone mineral density (BMD) measures bone mass, which is simply 1 component of bone strength. BMD does not assess bone microarchitecture, although it can facilitate a diagnosis of osteopenia or osteoporosis using the WHO definitions.

We use BMD to monitor risk of fracture, much as blood pressure predicts the risk of cardiovascular disease. Many patients with high blood pressure never have a heart attack or stroke, and many patients with normal blood pressure do—but overall, rising blood pressure and rising risk of cardiovascular disease go together.

We use BMD to monitor response to treatment, but it is accurate only if the concept of least-specific change (LSC) is taken into account: LSC=2.77×the precision error of the machine. Thus, in a good center, BMD measurement of the spine will be ±3%, and measurement of the hip will be ±5%.

Bone loss is a continuum, not a T score

Another limitation of the term osteopenia: There is a lot of distance under the curve from –1 to –2.49 standard deviations. Thus, when it comes to risk assessment, it is important to remember that loss of bone mass is a continuum. And because the risk of fracture is directly related to bone mass, fracture risk is a continuum, too. For every standard deviation of bone mass lost, the relative risk of fracture doubles, but absolute fracture risk is highly age-dependent (FIGURE 1).

In younger women, the relative risk of fracture is quite low, and it remains low even when doubled.

However, as careful inspection of FIGURE 1 reveals, the absolute fracture risk of a 50-year-old with a T score of –3 (a score most clinicians would be very concerned about) is exactly the same as the absolute fracture risk of an 80-year-old woman with a T score of –1 (a score many clinicians might consider excellent for a woman that age).